Related
I test the fetch API with a callback, but my function returns "Promise State: Pending", and I don't understand why :
async function getJson(url, callback) {
await fetch(url)
.then(async function(response) {
return await response.json()
})
.then(function(data) {
console.log(data)
callback(data)
})
}
let getData = {
getAll: async function() {
await getJson('js/getJson/data.json', function(data) {
console.log(data.photographers) //OK
let test = data.photographers
return test
})
}
}
console.log(getData.getAll()); //return promise pending
Thanks
General advice for asynchronous and promise-based programming listed below...
Here's what getJson() should look like:
function getJson(url, callback) {
return fetch(url).then(function(response) {
return response.json();
});
}
Just return the promise that fetch() already returned - don't try to convert to a callback. Converting a promise-based interface to a callback interface is going backwards in terms of programming usability. Promises are much better to program with than plain callbacks for asynchronous programming.
Here's how getAll() can then use it:
const getData = {
getAll: async function() {
const data = await getJson('js/getJson/data.json');
return data.photographers;
}
}
Or, equally good (and equivalent to the above example) would be this:
const getData = {
getAll: function() {
return getJson('js/getJson/data.json').then(data => {
return data.photographers;
});
}
}
await has significant advantages when you are sequencing more than one asynchronous operation, but usually doesn't really help much when there's just one asynchronous operation. It's not wrong to use it then, it just doesn't really offer much help.
And, here's how one would call getAll():
getData.getAll().then(photographers => {
console.log(photographers);
}).catch(err => {
console.log(err);
});
General Advice and Explanation:
1. Read and study how async and await work with promises. Only use it once you understand that and then you will only be using await when you are awaiting a promise. await does nothing useful unless you are awaiting a promise so if you're awaiting a function call, then that function must be returning a promise and that promise must be connected to when the asynchronous operations in that function are completed.
2. Don't mix plain callbacks and promises. If you are programming with a promise interface, use that promise - never convert it to a plain callback. Return a promise from your function and let the caller use that. Among the many, many reasons that promises were invented is that control flow in non-simple asynchronous operations is massively simpler with promises (particularly asynchronous error handling and error propagation to higher levels).
3. Convert plain callbacks to promises. If you encounter an asynchronous operation that you want to use in a world where there are other promise-based asynchronous operations (such as fetch()), then wrap the plain callback into a promise interface so you are only mixing promise-based calls with other promise-based calls. Much, much simpler to code reliably.
4. async functions ALWAYS return a promise. That's how they are built internal to Javascript. So, a caller of an async function always gets back a promise as the return value. That promise will initially be in the pending state. It will be resolved sometime in the future and the eventual resolved value of the promise will be whatever value is returned from the outer scope of the async function. If there's no return statement in the outer scope of the async function, then the resolved value will be undefined as it is with both your `async functions.
5. A caller gets a resolved value from a promise with .then() or await. Those are the only two ways to get a resolved value out of a promise. So, any caller of an async function that wants some value back from it needs to use .then() or await to get that value.
6. If you have a promise-based operation inside a function and you wish to return it's resolved value from your function, then just return that promise from the function. That will allow the caller to use that promise to get the value. See my getJson() implementation above for how simple that can be.
7. Avoid return await fn() and use return fn() instead. If you're using return await fn(), then you're already in an async function and thus the function is already returning a promise. So, avoid the extra await as it doesn't do anything useful and just use return fn(). If fn() returns a value that value will become the resolved value of the promise that your async function returned. If fn() returns a promise, then the resolved value of that promise will become the resolved value of the promise that your async function returned.
8. Returning a value from a .then() handler becomes the resolved value of the parent promise. In the second getData() example above that uses .then() internally, the return data.photographers; statement sets the resolved value of the parent promise to data.photographers. So, any caller of getData() will find that data.photographers becomes the resolved value of the promise that getData() returns.
9. Returning a promise from a .then() handler chains the promises and the resolved value of the promise you return becomes the resolved value of the parent promise. Essentially, returning a promise from a .then() causes the parent promise to wait for the newly returned promise to resolve and it then gets its resolved value from that newly returned promise. You can see this in play in the getJson() function where response.json() returns a new promise that resolves to the json-parsed body of the http request. That resolved value will become the resolved value of the promise that the function returned.
10. Don't pass a callback when expecting a promise back. If you're passing a callback to some asynchronous function, then most of the time that function will not be returning a promise because many asynchronous APIs these days accept either a callback or return a promise, but don't do both at the same time. So, when looking to use await, make absolutely sure the function you're awaiting is returning a promise. When in doubt, look at the doc. If the doc is unclear look at the code for the function itself or run an experiment to see what the return value actually is. As an example, most of the mongodb asynchronous APIs will return a promise if you do NOT pass a callback to them, but will not return a promise if you do pass the callback. Use one or the other, not both.
It works, although it's not really what I wanted. Because I thought I could store my result outside of functions in a variable. But it seems that this is not possible in fact.
All this and due to the fact that I have to give an evaluation for my training. Fetch is not mandatory.
In the first version of my code, I didn't use it. I just did classes and function synchronously by loading my JSON into a variable and adding it to a script tag.
However, I wanted to do some tests because then I have to use the design pattern factory method. And I just tested and it works with your code. Thank you for taking the time to respond to me at such a late hour.
//we instantiate the factory
let factory = new identityFactory
getData.getAll().then(photographers => {
let identity = photographers
console.log(identity);
//we pass the identity in the factory
let newIdentity = factory.createIdentity(identity,'all')
console.log(newIdentity);
showIdentity(newIdentity)
}).catch(err => {
console.log(err);
});
I want to fulfill a promise with some other promise. The point is that I really want to get access to the (still pending) second promise as soon as the first promise is fulfilled. Unfortunately, I only seem to be able to get the second promise's resolution value once both both promises are fulfilled.
Here's the use case that I have in mind:
var picker = pickFile();
picker.then( // Wait for the user to pick a file.
function(downloadProgress) {
// The user picked a file. The file may not be available just yet (e.g.,
// if it has to be downloaded over the network) but we can already ask
// the user some more questions while the file is being obtained in the
// background.
...do some more user interaction...
return downloadProgress;
}
).then( // Wait for the download (if any) to complete.
function(file) {
// Do something with the file.
}
)
The function pickFile displays a file picker where the user may pick a file either from their own hard drive or from a URL. It returns a promise picker that is fulfilled as soon as the user has picked a file. At this point, we may still have to download the selected file over the network. Therefore, I cannot fulfill picker with the selected file as resolution value. Instead, picker should be fulfilled with another promise, downloadProgress, which in turn will eventually be fulfilled with the selected file.
For completenes, here's a mock implementation of the pickFile function:
function pickFile() {
...display the file picker...
var resolveP1 = null;
var p1 = new Promise(
function(resolve, reject) {
resolveP1 = resolve;
}
);
// Mock code to pretend the user picked a file
window.setTimeout(function() {
var p2 = Promise.resolve('thefile');
resolveP1(p2); // <--- PROBLEM: I actually want to *fulfill* p1 with p2
}, 3000);
return p1;
}
The problem in the marked line is that I would like to fulfill the promise p1 with the new promise p2, but I only know how to resolve it. The difference between fulfilling and resolving is that resolving first checks if the supplied value p2 is again a promise. If it is, then fulfillment of p1 will be deferred until p2 is fulfilld, and then p1 will be fulfilled with p2's resolution value instead of p2 itself.
I could work around this issue by building a wrapper around p2, i.e. by replacing the line
resolveP1(p2); // <--- PROBLEM: I actually want to *fulfill* p1 with p2
from the second code example by
resolveP1({promise: p2});
Then, in the first code example, I'd have to replace the line
return downloadProgress;
by
return downloadProgress.promise;
But this seems like a bit of a hack when all I really want to do is just fulfill (instead of resolve) a promise.
I'd appreciate any suggestions.
There doesn't seem to be a solution apart from the workaround I already described in the question. For future reference, if you want to fulfill (rather than resolve) a promise p with a value val, where val is another promise, then just calling the promise resolution function for p with argument val won't work as expected. It would cause p to be "locked in" on the state of val, such that p will be fulfilled with val's resolution value once val is fulfilled (see spec).
Instead, wrap val in another object and resolve p with that object:
var resolveP; // Promise resolution function for p
var p = new Promise(
function(resolve, reject) {
resolveP = resolve;
}
);
function fulfillPwithPromise(val) { // Fulfills p with a promise val
resolveP({promise: val});
}
p.then(function(res) {
// Do something as soon as p is fulfilled...
return res.promise;
}).then(function(res) {
// Do something as soon as the second promise is fulfilled...
});
This solution works if you already know that val is a promise. If you cannot make any assumptions about val's type, then you seem to be out of luck. Either you have to always wrap promise resolution values in another object, or you can try to detect whether val has a field then of type "function" and wrap it conditionally.
That said, in some cases the default behavior of promise resolution may actually have the desired effect. So only use the workaround described above if you are sure that you want to fulfill instead of resolve the first promise with the second one.
Although different people use different terms, in common terminology, "fulfill" means to put a promise in the "success" state (as opposed to "reject")--the state that will trigger then then handlers hanging off it.
In other words, you cannot "fulfill" a promise with a promise. You can fulfill it with a value. (By the way, the term "resolve" is usually meant as either of fulfilling or rejecting.)
What you can do is return a promise from a .then handler and that will have the effect of essentially replacing the original promise with the returned promise.
Here is a simple example of doing that:
asyncTask1 . then(asyncTask2) . then(processData)
where asyncTask1 is a promise, and asyncTask2 is a function which returns a promise. So when asyncTask1 is fulfilled (done successfully), then asyncTask2 runs, and the promise returned by the .then is "taken over" by the promise asyncTask2 returns, so that when it finishes, the data can be processed.
I can do something similar by calling Promise.resolve with a promise as parameter. It's a bit of a misnomer, because I'm not resolving the promise in the technical sense. Instead, the new promise created is "inhabited" by the promise I passed in. It's also useless, because using the result is exactly the same as using the promise I passed in:
Promise.resolve(asyncTask2)
behaves exactly the same as
asyncTask2
(assuming asyncTask2 is already a promise; otherwise Promise.resolve has the effect of creating a promise which is immediately fulfilled with the passed in value.)
Just as you can pass a promise to Promise.resolve, you can pass a promise to the resolve function provided to you as a parameter of the promise constructor callback. If the parameter you pass to resolve is a non-promise, the promise immediately fulfills with that value. However, if the parameter you pass to resolve is another promise, that promise "takes over the body" of the promise you are constructing. To put it another way, the promise you are constructing starts to behave exactly as the the promise passed to resolve.
By "behave exactly" I mean, if the promise you pass in to resolve is already fulfilled, the promise you are constructing is instantly fulfilled with the same value. If the promise you pass in to resolve is already rejected, the promise you are constructing is instantly rejected with the same reason. If the promise you pass in to resolve is not resolved yet, then any then handlers you hang off the promise you are constructing will be invoked if and when the promise you pass to resolve is resolved.
Just as it is confusing that Promise.resolve may result in a promise which is not actually resolved, it is similarly confusing that calling the resolve function handed to you as a parameter to the promise constructor may not actually resolve the promise being constructed if you call it with an unresolved promise. Instead, as I've said a couple of times now, it has the effect of putting the promise being constructed in a state of total congruence with the promise passed to resolve.
Therefore, unless I am missing the point of your question, pickfile could be written as
function pickFile() {
return new Promise(function(resolve, reject) {
...display the file picker...
// Mock code to pretend the user picked a file
window.setTimeout(function() {
resolve('thefile');
});
}
I didn't really understand your question clearly, so this might not be what you want. Please clarify if you care to.
Found a similar solution in the process of moving away from Angular's $q to the native Promise feature. Promise.all could be an option (in cases of independent parallel async tasks) by passing around an appropriate object, or something decorated with the state, passing it off to whatever is ready when appropriate. In the Promise.all sample below note how it recovers in one of the promises--took me awhile to realize how to redirect the result of a chain. The result of the all is just the last promise's return. While this doesn't answer the question's title, using return Promise.reject(<an-object-including-a-promise>) (or resolve) gives a series and/or group of async tasks shared access and control along the way. In the case of picking, downloading then working with a file I'd take out the progress-event handling then do: pickFile.then(download,orFailGracefully) with downloadProgress handled within the download onResolve handler (download-progress doesn't appear to be an async task). Below are related experiments in the console.
var q = {
defer: function _defer(){
var deferred = { };
deferred.promise = new Promise(function(resolve, reject){
deferred.resolve = resolve;
deferred.reject = reject;
});
return deferred;
}
};
var communityThatCares = q.defer();
communityThatCares.promise.then(function(someGood){
console.log('someGood', someGood);
return someGood;
}, function(someBad){
console.warn('someBad', someBad);
return someBad;
});
(new Promise(function(resolve, reject){ communityThatCares.about = 'communityThatCares'; setTimeout(resolve,1000, communityThatCares); }))
.then(
function(e){
console.log(3,e); return e.resolve(e);
}, function(e){
console.warn(3, e); return e.reject(e);
});
var todo = {
find:'swan'
};
var greaterGood = [(
(new Promise(function(res,rej){ res(todo); })).then(function(e){ e.stuff = 'things'; return e; }),
(new Promise(function(res,reject){
reject(todo);
})).then(function(e){ return e; }
,function(e){
console.warn(1,e);
e.recover = 'uh oh';
return Promise.resolve(e);
}).then(function(e){ console.log(2,e); return e; }),
(new Promise(function(res,rej){ res(todo); })).then(function(e){ console.log(1,e); e.schedule = 'today'; return e; },function(e){ console.warn(1,e); return e; }).then(function(e){ console.log(2,e); return e; }))
];
var nkay = Promise.all( greaterGood )
.then(function(todo){
console.log('all',todo[0]); return todo;
}, function(todo){
console.warn('all',todo[0]); return todo;
});
The accepted answer for How do I convert an existing callback API to promises? has this example:
function getUserDataAsync(userId){
return new Promise(function(resolve,reject){
getUserData(userId,resolve,reject);
});
}
Questions:
Shouldn't the body of Promise return something, not just call getUserData (at least in practical code, not as promise demonstration exercise)?
Is this particular Promise also a closure? It seems like it closes over userId, as getUserDataAsync creates and returns a new Promise instance using userId, but I want to be sure.
Shouldn't the body of Promise return something, not just call getUserData (at least in practical code, not as promise demonstration exercise)?
No, it doesn't have to. In fact, returning a value from the Promise constructor will have no effect. Only resolve and reject functions can fulfill a Promise.
Is this particular Promise also a closure? It seems like it closes over userId, as getUserDataAsync creates and returns a new Promise instance using userId, but I want to be sure.
Yes, the function passed to the Promise constructor closes over userId.
No, it shouldn't return something, because there is nothing to return at that point in time, hence the reason you are using a promise.
Yes.
I would like to get a deeper understanding of how Promises work internally.
Therefore I have some sample code:
var p1 = new Promise(
function(resolve, reject) {
window.setTimeout(
function() {
resolve('res called')
}, 2000);
});
var p2 = new Promise(
function(resolve, reject) {
window.setTimeout(
function() {
resolve('res called')
}, 2000);
});
function chainPromises() {
return p1.then(function(val) {
console.log("p1");
return p2.then(function(val) {
console.log("p2");
return val;
});
});
}
chainPromises().then(function(val) {
console.log(val);
});
Here a link to execute this code.
As you would predict, first p1 is resolved, afterwards p2 and in the end the final then prints the resolv value.
But the API ref states the following:
"then" returns a new promise equivalent to the value you return from
onFulfilled/onRejected after being passed through Promise.resolve
So it would be interesting to know WHEN exactly the "then" function is executed?
Because the final "then" in the code is chained to the chainPromises(), I first thought that
it would execute after the function chainPromises() returns something (in this case another promise).
If this would have been the case the "val" of the final "then" function would be the returned promise.
But instead, the final "then" waits until all promises inside the first "then" which are returned have been resolved.
This absolutely makes sense because in this way, the "then" functions can be stacked, but
I do not really get how this is done, since the API spec. does not really cover what "then" returns and when the "then" functions is executed.
Or in other words, why does the final "then" function wait until all the Promises are resolved inside the chainPromises() function instead of just waiting for the first returned object as the API doc says.
I hope I could make clear what I mean.. :)
About Promise resolution
The thing you're witnessing here is called recursive thenable resolution. The promise resolution process in the Promises/A+ specification contains the following clause:
onFulfilled or onRejected returns a value x, run the Promise Resolution Procedure [[Resolve]](promise2, x)
The ES6 promise specification (promises unwrapping) contains a similar clause.
This mandates that when a resolve operation occurs: either in the promise constructor, by calling Promise.resolve or in your case in a then chain a promise implementation must recursively unwrap the returned value if it is a promise.
In practice
This means that if onFulfilled (the then) returns a value, try to "resolve" the promise value yourself thus recursively waiting for the entire chain.
This means the following:
promiseReturning().then(function(){
alert(1);
return foo(); // foo returns a promise
}).then(function(){
alert(2); // will only run after the ENTIRE chain of `foo` resolved
// if foo OR ANY PART OF THE CHAIN rejects and it is not handled this
// will not run
});
So for example:
promiseReturning().then(function(){
alert(1);
return Promise.resolve().then(function(){ throw Error(); });
}).then(function(){
alert("This will never run");
});
And that:
promiseReturning().then(function(){
alert(1);
return Promise.resolve().then(function(){ return delay(2000); });
}).then(function(){
alert("This will only run after 2000 ms");
});
Is it a good idea?
It's been the topic of much debate in the promises specification process a second chain method that does not exhibit this behavior was discussed but decided against (still available in Chrome, but will be removed soon). You can read about the whole debate in this esdiscuss thread. This behavior is for pragmatic reasons so you wouldn't have to manually do it.
In other languages
It's worth mentioning that other languages do not do this, neither futures in Scala or tasks in C# have this property. For example in C# you'd have to call Task.Unwrap on a task in order to wait for its chain to resolve.
Let's start with an easy perspective: "chainPromises" returns a promise, so you could look at it this way:
// Do all internal promises
var cp = chainPromises();
// After everything is finished you execute the final "then".
cp.then(function(val) {
console.log(val);
});
Generally speaking, when returning a promise from within a "then" clause, the "then" function of the encapsulating promise will be marked as finished only after the internal "then" has finished.
So, if "a" is a promise, and "b" is a promise:
// "a"'s "then" function will only be marked as finished after "b"'s "then" function has finished.
var c = a.then(function () {
return b.then(function () {
console.log("B!");
};
};
// c is a promise, since "then" always returns a promise.
c.then(function() {
console.log("Done!");
};
So the output will be:
B!
Done!
Notice btw, that if you don't "return" the internal promise, this will not be the case:
// "a"'s "then" function will only be marked as finished without waiting for "b"'s "then" to finish.
var c = a.then(function () {
// Notice we're just calling b.then, and don't "return" it.
b.then(function () {
console.log("B!");
};
};
// c is a promise, since "then" always returns a promise.
c.then(function() {
console.log("Done!");
};
Here we can't know what would be outputted first. It could be either "B!" or "Done!".
Please check the below example regarding how promises works:
The Promise object represents the eventual completion (or failure) of an asynchronous operation, and its resulting value.
console.log('person1: shoe ticket');
console.log('person2: shoe ticket');
const promiseGirlFriendBringingTickets = new Promise((resolve, reject) => {
setTimeout(() => {
resolve('ticket');
}, 3000);
});
promiseGirlFriendBringingTickets.then((t) => {
console.log(`person3: show ${t}`);
})
console.log('person4: shoe ticket');
console.log('person5: shoe ticket');
Promise then return promise object, not promise's resolved value. I forked your JsFiddle, and added some of mine try this.
promise.then is executed right after that promise object is resolved.
I do not know how this is done in actual promises libraries, but I was able to re-create this functionality in the following way:
1) each promise has a waitingPromises property;
2) then method returns a new promise, and the original promise's waitingPromises property points to the new promise.
In this way, the chain of .then()s creates a structure that is similar to a linked list or rather a tree (each promise can have several waiting promises). A promise can be resolved only after its 'parent' promise has been resolved. The .then method itself is executed immediately, but the corresponding promise that it creates is resolved only later.
I am not sure this is a good explanation and would love to learn about other possible approaches.
Normally code is synchronous - one statement executes like (fileopen) and there is a guarantee that the next statement will execute immediately afterwards like filewrite()
but in asynchronous operations like nodejs, you should assume that
you have no idea when the operation will complete.
You can't even assume that just because you send out one request first, and another request second, that they will return in that order
Callbacks are the standard way of handling asynchrnous code in JavaScript
but promises are the best way to handle asynchronous code.
This is because callbacks make error handling difficult, and lead to ugly nested code.
which user and programmer not readble easily so promises is the way
You can think of Promise as a wrapper on some background task. It takes in a function which needs to be executed in the background.
The most appropriate place to use a promise is where some code is dependent on some background processing and it needs to know the status of the background task which was executed. For that, the background task itself accepts two callback resolve and reject in order to convey its status to the code which is dependent on it. In layman terms, this code is the one behind it in the promise chain.
When a background task invokes resolve callback with some parameter. it's marking the background operation successful and passing the result of the background operation to the next then block which will be executed next. and if it calls reject, marking it as unsuccessful then the first catch block will be executed.
In your custom promise, you can pass an error obj to the reject callback so that next catch block is aware of the error happened in the background task.
I have a number of async tasks that need to be completed, so I'm using promises.
I need to detect when each one of the promises has been executed (both resolved and rejected). I must not continue execution until that point.
I was using something like this:
$.when(promise1, promise2, ...).always();
But this code is wrong, because the when method has lazy evaluation, and it returns as soon as one of the promises fails. So the always callback also runs as soon as one of the promises fail.
I was thinking in coding a workaround, but this use case is so common that maybe somebody has done it already, or maybe there's even a way of doing this using just jQuery (if not, it would be nice to add a Promise.whenNonLazy or a Promise.when(promise1, promise2, ..., false) in the future.
Is this possible?
More sophisticated promise libraries have an allSettled() function like Q or Promise.settle like Bluebird.
In jQuery, you could implement such a function yourself as well and extend the $ namespace with it, but that will only be necessary if you need it often and performance-optimized.
A simpler solution would be to create a new promise for each of the ones you are waiting for, and fulfilling them even when the underlying one is rejected. Then you can use $.when() on them without problems. In short:
// using Underscore's .invoke() method:
$.when.apply(null, _.invoke(promises, "then", null, $.when)).done(…)
More stable:
$.when.apply($, $.map(promises, function(p) {
return p.then(null, function() {
return $.Deferred().resolveWith(this, arguments);
});
})).then(…);
You might change the then callbacks a bit to distinguish between fulfilled and rejected results in the final done.
Smithy,
First let's assume your promises are in an array.
var promises = [....];
What you appear to want is .when() applied to some transform of these promises, such that any rejected promise is converted to resolved, whilst being transparent to promises that are already resolved.
The required operation can be written very succinctly as follows :
$.when.apply(null, $.map(promises, resolvize)).done(...);
//or, if further filtering by .then() is required ...
$.when.apply(null, $.map(promises, resolvize)).then(...);
where resolvize is the transform mechanism.
So what should resolvize(), look like? Let's exploit the characteristics of .then() to make the distinction beteween a resolved and a rejected promise, and respond accordingly.
function resolvize(promise) {
//Note: null allows a resolved promise to pass straight through unmolested;
return promise.then(null, function() {
return $.Deferred().resolve.apply(null, arguments).promise();
});
}
untested
With resolvize in some outer scope, it can be made available to be used in a $.when.apply($.map(promises, resolvize)) expression wherever it is needed. This is most likely adequate, without going to the extent of extending jQuery with a new method.
Regardless of how the transform is achieved, you end up with a potential issue; namely knowing for each argument of the .done() callback, whether its corresponding promise was originally resolved or rejected. That's the price you pay for converting rejection to resolution. You may, however, be able to detect the original status from the parameter(s) with which the original promises were resolved/rejected.
That's an interesting property of always - I hadn't expected that behaviour.
I suppose you could use a master, top-level deferred to monitor the states of the main deferreds, which is resolved only once the main deferreds are all either resolved or rejected. Something like:
//set up master deferred, to observe the states of the sub-deferreds
var master_dfd = new $.Deferred;
master_dfd.done(function() { alert('done'); });
//set up sub-deferreds
var dfds = [new $.Deferred, new $.Deferred, new $.Deferred];
var cb = function() {
if (dfds.filter(function(dfd) {
return /resolved|rejected/.test(dfd.state());
}).length == dfds.length)
master_dfd.resolve();
};
dfds.forEach(function(dfd) { dfd.always(cb); });
//resolve or reject sub-deferreds. Master deferred resolves only once
//all are resolved or rejected
dfds[0].resolve();
dfds[1].reject();
dfds[2].resolve();
Fiddle: http://jsfiddle.net/Wtxfy/3/